Fluorescent lamp and method for the manufacture thereof

ABSTRACT

A fluorescent lamp having a phosphor coating on the inside surface of the tubular glass envelope, the phosphor coating comprising finely divided phosphor particles coated with a glassy phosphate material. The lamp is prepared by providing an aqueous slurry of the phosphor particles in a solution containing a material yielding a phosphate glass upon firing, coating the interior envelope walls with the aqueous suspension, and firing the phosphor coating at a temperature which promotes the formation of the phosphate glass coating about the particulate phosphor material. A suitable phosphate glass-yielding material is ammonium dihydrogen phosphate, and phosphor coatings produced therewith have been found to have improved adherence to the envelope walls.

This is a division, of application Ser. No. 364,568 filed May 29, 1973now U.S. Pat. No. 3,890,522.

BACKGROUND OF THE INVENTION

This invention relates to fluorescent lamps and to methods for thefabrication thereof. More particularly, this invention relates tofluorescent coatings applied to the interior surfaces of the envelopesof such lamps, and to methods for applying such fluorescent coatings.Specifically, this invention relates to aqueous suspensions of phosphorparticles, and the improvement in the adherence of fluorescent coatingswhich can be attained therewith when such suspensions are applied to theinterior surface of the envelope of a fluorescent lamp.

Commercially available fluorescent lamps comprise an elongated tubularenvelope having a pair of electrodes sealed into the opposite endsthereof. The envelope contains in gaseous atmosphere, which may be amixture of a rare gas and a metal vapor, such as mercury vapor. Theinterior surface of the envelope is coated with a finely-dividedfluorescent material which is exposed to the discharge between theelectrodes, and is excited by ultra-violet radiations emitted by thisdischarge. The fluorescent coating is usually applied by suspendingparticulate fluorescent or phosphor material in a suitable binder,flushing the interior of the envelope with the suspension, permittingthe excess suspension to drain out of the envelope, and then firing theinterior wall of the coated envelope at a temperature which promotesadherence of the coating to the envelope walls and removes, generally byvolatilization, the binder material. There results a phosphor film orcoating adhered to the inside or interior surfaces of the tubularenvelope.

Thus, in the course of manufacture, as well as during operation, ofthese fluorescent lamps, the phosphors used in these lamps experience ahostile environment. During manufacture, the lamp is baked attemperatures approximating 600°C which can cause serious degradation ofsome phosphors. While the lamp is operating, the phosphor is in amercury vapor discharge where it is exposed to high-energy ultra-violetradiation, and is bombarded by electrons and mercury atoms. Thesefactors may be responsible for maintenance losses, i.e., thetime-dependent decrease in luminous flux found in all fluorescent lamps.

One method for improving brightness and maintenance is to shield thephosphor from these effects by coating the phosphor particles with aprotective film, and numerous attempts have been made to protectivelycoat the particulate phosphor material by using selected additions tothe coating suspension. Such additives include silica, boric anhydride,sodium borate, mixtures of barium and ammonium nitrates, crystallinealkali halides, calcium pyrophosphate, etc. These materials have severaldisadvantages as lamp phosphor coatings. The polycrystalline additions,by their very nature, cannot be expected to form an impervious film onthe phosphors, therefore protection is incomplete. Of the glassymaterials utilized, silica requires a very high temperature before it isfluid enough to coat the phosphor particles, whereas the other materialsare not chemically compatible with most lamp phosphors and are not veryresistant to attack by mercury vapor. In addition, since the phosphorsare excited by the 254-nm line of the mercury discharge, theultra-violet absorption of these glasses must be considered. Forexample, although boric anhydride, in the pure state, is quitetransparent to ultraviolet radiation, additions, such as sodium whichcan migrate from the glass tubular envelope wall, cause the absorptionedge to shift to longer wavelengths which could lead to absorption ofthe radiation required to excite the phosphor material.

In addition, it is well known that there is a tendency of thefluorescent coatings to chip or flake off the inside surface of thetubular envelope either during the manufacturing operation or duringuse. Various means have been proposed for improving the adherence of thefluorescent coating to the envelope walls. However, such techniques ashave been proposed to date for use in conjunction with aqueoussuspensions of the phosphor particles have not been completelysatisfactory for a variety of reasons.

OBJECTS OF THE INVENTION

It is, therefore, the object of this invention to provide an improvedfluorescent lamp.

It is a further object of this invention to provide a fluorescent lamphaving a fluorescent coating firmly adhering to the interior wall of thetubular envelope.

It is a further object of this invention to provide a fluorescent lamphaving a glassy phosphate coating on the phosphor particles whichprotects them from the environment in the lamp during use.

It is a further object of the present invention to provide a method forcoating the interior wall of a tubular fluorescent lamp envelope with afluorescent coating which improves the adherence of the phosphor to theenvelope wall with little or no loss of brightness.

It is a further object of the invention to provide a method for coatingphosphor particles on the interior wall of a tubular fluorescent lampenvelope in a manner which protects the phosphor particles fromdegradation that normally occurs during fabrication and operation ofsuch a fluorescent lamp, but which will allow the phosphor to be fullyexcited by incident ultra-violet radiation generated during use.

Another object of this invention is to provide a novel aqueoussuspension of phosphor particles suitable for coating the interiorsurface of the tubular envelope of a fluorescent lamp.

Yet a still further object of the present invention is to provide anaqueous suspension of phosphor particles and selected additive materialswhich will, in use according to the method of the present invention,provide a glassy phosphate coating on the phosphor particles to protectsuch particles from the environment generated in the fluorescent lampduring periods of use.

These and still further objects, features and advantages of the presentinvention will become apparent upon consideration of the followingdetailed disclosure.

BRIEF SUMMARY OF THE INVENTION

These and still further objects, features and advantages of theinvention are achieved, in accordance therewith, by providing an aqueoussuspension of phosphor particles containing an additive which will yielda phosphate glass upon firing. A suitable additive is, for example,ammonium dihydrogen phosphate. Other suitable additives includeorthophosphoric acid, and the like. In order to obtain good adherence ofthe fluorescent coating to the envelope wall with minimum loss inbrightness, the amount of the additive added to the aqueous suspensionshould be in the range from about 0.01 gram to about 0.5 gram of theadditive (e.g., ammonium dihydrogen phosphate), preferably about 0.05grams additive, per 100 grams of the phosphor material. The viscosity ofthe resultant suspension should be such that the material can beuniformly coated on the interior envelope wall, for example, about 90centipoise.

During manufacture of the fluorescent lamp, the tubular envelope, afterbeing coated with the aforementioned aqueous suspension, is dried, andthen baked at approximately 600°C for a period of time sufficient toform a glassy phosphate film on the phosphor particle surface, generallyfor about one minute. It has been found that the manufacture offluorescent lamps with the aqueous suspension described above, inaccordance with the method described above, results in lamps havingimproved adherence of the phosphor coating to the fluorescent tube wallwith little or no loss of brightness and, in some cases, an actualincrease in the brightness of the treated phosphors.

In addition to the improved adherence of the phosphor coating to theenvelope wall and the protection afforded to the particulate phosphormaterial by the glassy phosphate layer or matrix, a further advantage ofthe present invention is that these beneficial results can be attainedwith no additional processing steps. That is, by use of the aqueoussuspensions herein described in place of prior art coating suspensions,according to conventional fluorescent lamp manufacturing techniques, thebeneficial results described above are attained.

In one embodiment (see Example IV and V below), a ZnO - P₂ O₅ glasscoating is prepared by utilizing a combination of orthophosphoric acidand zinc nitrate. The atomic ratio of zinc to phosphorus can be between0.1 to 0.5 with a presently preferred value of about 0.25 for calciumhalophosphate phosphor. The amount of additive should be between 2 × 10⁻³ and 2 × 10⁻ ⁴ total moles H₃ PO₃ and Zn (NO₃)₂ .sup.. 6H₂ O per 100 g.phosphor. The presently preferred amount for additions to calciumhalophosphate phosphor is about 9 × 10⁻ ⁴ total moles H₃ PO₄ andZn(NO₃)₂ .sup.. 6H₂ O per 100 g. phosphor. As with other suspensionsdescribed herein, the viscosity of the suspension should be, or adjustedto be, about 90 centipoise.

The suspensions of this invention have a relatively major amount ofwater, for example, about 50 to about 80 ml. per 100 grams of phosphor,a relatively major amount of the temporary vehicle, for example, about60 to about 70 ml. per 100 grams of phosphor, and a minor amount of theadditive as set forth above.

The present invention has numerous advantages over known, priorsuspension coating techniques. Initially, the coating obtained herewithis chemically compatible with most lamp phosphors, and does not affectthe oxidation state of activators-- in fact, the coating actuallyimproves the degradation resistance during processing of phosphors withreduced valence state activators. The coating, consisting as it does ofa low melting phosphate glass, easily forms an impervious layer thatresists attack by metallic vapors, such as mercury vapor present influorescent lamps. The coating is transparent to ultra-violet radiationand, moreover, any impurities in the coating tend to improve itsultra-violet transmission, thus allowing full excitation by theultra-violet radiation from the mercury discharging. Finally, asindicated above, the coating, being a low melting glass, aids in theadherence of the particulate phosphor material to the inside surface ofthe envelope wall, and these beneficial results can be attained by theherein described technique which requires no change in commercial lampmanufacturing production lines, other than the use of the aqueoussuspensions herein described.

Since the vehicle for the suspension is water, the suspension is lesshazardous to use than organic vehicle suspensions, based upon, forexample, xylene, which is flammable and sufficiently toxic thatbreathing of its vapors and contact with the skin should be avoided.

The fluorescent lamps of the present invention can have the structure asshown, for example, in U.S. Pat. Nos. 2,151,496; 3,424,605; 3,424,606;or 3,435,271; the Figures of which, and the portions of thespecification corresponding thereto, are incorporated herein byreference to the extent necessary to complete this specification. Suchfluorescent lamps consist of a hermetically sealed tubular glassenvelope coated on its inside surface with the powdered phosphor coatingin accordance with the present invention, which converts theultra-violet energy of a mercury arc discharge established through thecenter of the tube into visible light as it is absorbed by the phosphorlayer during operation of the lamp. The bases at each end of the tubularenvelope support electrical leads which are electrically connected toelectrodes at each end of the lamp. The spaced electrodes can be coatedwith electron-emission promoting materials, such as mixtures of oxidescontaining a barium oxide, to facilitate operation of the lamp. Duringoperation, a mercury droplet maintained within the sealed envelope isvaporized thereby causing the characteristic mercury discharge. Exceptfor the nature of the phosphor coating, which is the subject of thepresent invention, construction of the fluorescent lamp is conventional.

The present invention also has applicability to the high pressuremercury vapor lamp shown in FIG. 2 of U.S. Pat. No. 3,435,271. Such alamp has a radiation-transmitting outer envelope having a conventionalscrew-type base connected thereto. The improved adherent phosphorcoating of the present invention can be coated on the inner surface ofthe outer envelope. An arc tube or inner sealed envelope can besupported within the outer envelope by means of a conventionalsupporting frame. A starting resistor connects a starting electrode ofone side of the energizing potential, and, during operation, the mercuryarc is sustained within the arc tube between spaced operating electrodesat each end of the arc tube. As with the fluorescent lamp described inthe preceding paragraph, the construction of the lamp of this paragraphis conventional, with the exception of the coated phosphor layer whichforms the subject matter of the present invention.

Any suitable phosphor can be utilized in the practice of the presentinvention. For example, halophosphates of alkaline earth metals, such ascalcium, and activated with antimony alone or both antimony andmanganese are suitable phosphor materials. Other suitable phosphorsinclude, for example, zinc silicate activated with manganese, bariumtatanium phosphate, etc.

The components of the suspension, including the phosphor particles, thebinder, the phosphate-glass-yielding additive and the liquid componentsof the vehicle or carrier, are thoroughly mixed together, flowed overthe inner surface of the glass envelope, dried and baked in aconventional manner at a temperature, generally about 600°C, for a fewminutes, generally about one minute, in a manner sufficient to form aphosphate glass matrix or coating about, or around, the individualphosphor particles. The tubular envelope is then made into a lamp, inthe usual manner, by sealing alkaline earth oxide activated, filimentarytungsten electrodes into the ends thereof, evacuating the envelope,filling the envelope with a small quantity of mercury and a startinggas, such as argon, at a few millimeters pressure, and then sealing theend(s) thereof.

The existence of the phosphate glassy coating about the particulatephosphor material has not actually been determined to be present byspecific analysis. However, the improved adherence of the phosphorcoatings to the envelope walls cannot be explained in the absence ofsuch a glassy coating. Accordingly, the improved adherence is consideredto be an indication that the desired phosphate glassy coating is, infact, present.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The following Examples are given to enable those skilled in this art tomore clearly understand and practice the present invention. They shouldnot be considered as a limitation upon the scope of the invention butmerely as being illustrative and representative thereof.

In the following Examples, adherence of the phosphor coating to thecoated substrate is evaluated by placing the coated substrate at a setand predetermined distance from a regulated air line, increasing thepressure of the air flow, and noting the air pressure when the coatingis blown off. Brightness (luminosity) is measured by excitation with afiltered low-pressure mercury vapor lamp.

The phosphor numbers given in the Examples below are the identifyingnumbers utilized by GTE Sylvania Incorporated, from which company thephosphors were obtained.

EXAMPLE I

An aqueous phosphor coating suspension is prepared having variousamounts of ammonium dihydrogen phosphates, as given below in Table I, 50ml. distilled water and 66 ml. of the vehicle of Example VI per 100grams of calcium halophosphate phosphor (cool white No. 4467). Thephosphor is uniformly dispersed in the aqueous vehicle by bead millingwith glass beads and the viscosity adjusted, with distilled water ifnecessary, to about 90 centipoise. The suspension is coated onto theinterior surfaces of a tubular fluorescent envelope, the excess aqueoussuspension material removed and the coated substrate fired at 600°C forabout 1 minute. The relative brightness (luminosity) and adherencevalues are given in Table I below.

                  TABLE I                                                         ______________________________________                                        Calcium Halophosphate (cool white, No. 4467)                                  Weight Percent                                                                              Luminosity    Adherence                                         NH.sub.4 H.sub.2 PO.sub.4                                                                   (Relative)    (Relative PSI)                                    ______________________________________                                        0             100           100                                               0.01          100           110                                               0.10           97           170                                               0.50           98           >400                                              ______________________________________                                    

EXAMPLE II

An aqueous phosphor coating suspension is prepared having variousamounts of ammonium dihydrogen phosphate, as given below in Table II, 50ml. of distilled water and 66 ml. of the vehicle of Example VI per 100grams of zinc silicate:Mn (No. 2282). The phosphor is uniformlydispersed in the aqueous vehicle by bead milling with glass beads andthe viscosity adjusted, with distilled water if necessary, to about 90centipoise. The suspension is coated onto the interior surfaces of atubular fluorescent envelope, the excess aqueous suspension materialremoved and the coated substrate fired at 600°C for about 1 minute. Therelative brightness (luminosity) and adherence values are given in TableII below.

                  TABLE II                                                        ______________________________________                                        Zinc Silicate:Mn (No. 2282)                                                   Weight Percent                                                                              Luminosity    Adherence                                         NH.sub.4 H.sub.2 PO.sub.4                                                                   (Relative)    (Relative PSI)                                    ______________________________________                                        0             100           100                                               0.01          93            100                                               0.10          99            130                                               0.50          97            440                                               ______________________________________                                    

EXAMPLE III

An aqueous phosphor coating suspension is prepared having variousamounts of ammonium dihydrogen phosphate, as given below in Table III,50 ml. of distilled water and 66 ml. of the vehicle of Example VI per100 grams of barium titanium phosphate (No. 242). The phosphor isuniformly dispersed in the aqueous vehicle by bead milling with glassbeads and the viscosity adjusted, with distilled water if necessary, toabout 90 centipoise. The suspension is coated onto the interior surfacesof a tubular fluorescent envelope, the excess aqueous suspensionmaterial removed and the coated substrate fired at 600°C for about 1minute. The relative brightness (luminosity) and adherence values aregiven in Table III below.

                  TABLE III                                                       ______________________________________                                        Barium Titanium Phosphate (No. 242)                                           Weight Percent                                                                              Luminosity    Adherence                                         NH.sub.4 H.sub.2 PO.sub.4                                                                   (Relative)    (Relative PSI)                                    ______________________________________                                        0             100           100                                               0.01          100           110                                               0.10           98           150                                               0.50           82           390                                               ______________________________________                                    

EXAMPLE IV

An aqueous suspension is prepared having the stated total moles oforthophosphoric acid and zinc nitrate, as given in Table IV below, 50ml. of distilled water and 66 ml. of the vehicle of Example VI per 100grams of calcium halophosphate phosphor. The orthophosphoric acid andzinc nitrate are used in a molar ratio of 4 : 1. The phosphor isuniformly dispersed in the aqueous vehicle by bead milling with glassbeads and the viscosity adjusted, with distilled water if necessary, toabout 90 centipoise. A tubular fluorescent envelope is coated with theaforementioned suspension (which gives, upon baking, a ZnO-P₂ O₅ glasscoating having good transmission properties), dried and baked at 600°Cfor about one minute. The effect of the additive on relative adherenceof the fluorescent coating and brightness (luminosity) is given in TableIV below.

                  TABLE IV                                                        ______________________________________                                        Effect of the Amount of Addition (P/Zn = 4)                                   to Calcium Halophosphate Phosphor                                             Total Moles H.sub.3 PO.sub.3 +                                                             Approximate           Adherence                                  Zn(NO.sub.3).sub.2 per 100 g                                                               Weight Percent                                                                            Luminosity                                                                              (Relative                                  Phosphor × 10.sup.4                                                                  Addition    (Relative)                                                                              PSI)                                       ______________________________________                                        0            0           100       100                                        4.08         0.05        120       130                                        8.76         0.10        126       220                                        16.52        0.20        111       290                                        26.2         0.30        109       640                                        43.8         0.50        102       1100                                       ______________________________________                                    

EXAMPLE V

Example IV is repeated except the molar ratio of zinc nitrate toorthophosphoric acid is varied as given in Table V below. The relativeadherence and brightness (luminosity) values for such phosphor coatingsare also given in Tabel V below. The total number of moles oforthophosphoric acid and zinc nitrate is 16.52 × 10⁻ ⁴ moles.

                  TABLE V                                                         ______________________________________                                        Effect of the Composition of the Addition                                     (Total Addition about 0.2 Weight Percent)                                     Molar Ratio of                                                                              Luminosity    Adherence                                         Zn(NO.sub.3).sub.2 /H.sub.3 PO.sub.3                                                        (Relative),   (Relative PSI)                                    ______________________________________                                        0             100           100                                               0.11          101           130                                               0.25          107           230                                               0.50          102           250                                               ______________________________________                                    

In Examples IV and V, not only is there an improvement in the relativeadherence values but the relative brightness values increased as well,by as much as 26% in one instance, over the corresponding control. Thisis in comparison to the embodiments of Examples I-III where, althoughthere was substantial improvement in the relative adherence values, therelative luminosity values decreased, in most instances, byapproximately 1-3%, and, in other instances, by up to 18%. Totalluminous flux from the lamps of Examples IV and V is about 10% belowthat obtained from lamps prepared with a conventional organic (i.e.,xylene)-based suspension. Maintenance of these lamps, however, iscomparable to that of the conventionally prepared lamps.

EXAMPLE VI

A vehicle suitable for use in the present invention is composed of thefollowing components:

    Water                 6000    ml.                                             Isopropanol           1200    ml.                                             Polyox WSRN - 750     185     grams                                           (temporary organic binder)                                                    Carbonax 200 (Plasticizer)                                                                          50      ml.                                             Surfynol 104 (Anti-foaming                                                     agent)               15      grams                                       

EXAMPLE VII

A further vehicle suitable for use in the present invention is composedof the following components:

    Water                 6000    ml.                                             Isopropanol           1200    ml.                                             Polyox WSRN - 750     185     grams                                           Carbonax 200          50      ml.                                             Pluronic L61 (anti-foaming                                                     agent)               15      grams                                       

The present invention is considered separate and distinct from thematerials described in Great Britain Pat. No. 834,024, particularlyExample 3 thereof. The first formulation described in that Example 3does not have a temporary organic binder and thus is not suitable foruse as a suspension for coating the interior envelope walls of afluorescent lamp. As described, the first formulation is dried prior tothe addition thereto of the organic vehicle. Thus, the phosphor coatingsuspension of that Example is actually an organic vehicle-basedsuspension distinctly different from the aqueous suspension of thepresent invention, as set forth in the claims appended hereto.

While the invention has been described with reference to specificembodiments thereof, it should be understood by those skilled in thisart that various changes may be made without departing from the truespirit and scope of the invention. Various modifications may be made toadapt a particular situation, material, apparatus, process orthen-present objective to the spirit of this invention without departingfrom its essential teachings.

What is claimed is:
 1. An aqueous suspension comprising water; finely-divided phosphor particles; a temporary organic binder which can be essentially completely removed from a lamp envelope, on which the suspension has been coated, upon firing under appropriate conditions; and a minor amount of an additive which will yield a phosphate glass coating on said phosphor particles when a lamp envelope, on which the suspension has been coated, is fired under appropriate conditions, said additive comprising a mixture of orthophosphoric acid and zinc nitrate.
 2. The suspension of claim 1 wherein said orthophosphoric acid and said zinc nitrate are each present in amounts such that the atomicratio of zinc to phosphorus is about 0.1 to about 0.5.
 3. The suspension of claim 1 wherein about 0.002 - 0.0002 moles of said orthophosphoric acid, and 0.00004 - 0.0004 moles of said zinc nitrate are added per 100 grams of said phosphor particles.
 4. The suspension of claim 1 wherein said phosphate glass-yielding additive is present in an amount from about 0.01 gram to about 0.5 gram per 100 grams of said phosphor particles in said suspension.
 5. The suspension of claim 1 wherein the viscosity thereof is about 90 centipoise. 